The present invention relates to a signal processing system for suppressing background noise from signals containing discrete information, and more particularly to a stationary probability integrator system for processing underwater sounds having components of noise and spectral signatures to obtain maximum detection of submarines and other man-made sound sources.
LOFAR, which is an acronym for Low Frequency Analyzing And Recording, is one of several well-known ASW (anti-submarine warfare) techniques used to detect the spectral signatures of submarine-generated sounds which travel long distances in water. The complex waveforms of sound periodically sampled by a hydrophone suspended in the water are electrically transmitted to a spectrum analyzer which measures the amplitudes of the frequency components of each complex waveform throughout the frequency range of the waveform and produces an analog output of a series of voltage pulses, the amplitude of each pulse being indicative of the energy level in each frequency component. In practice, the voltage in each pulse represents the energy level for a range of frequency components called increments or "elements", and the frequency range per increment determines the resolution of each analysis. For a given resolution, the duration of each pulse and the frequency range of each entire spectrum analysis determines the sampling rate of the sound. The analyzer output may be displayed on a chart or cathode ray tube as an "A" or "B" trace. An "A" trace has horizontal and vertical coordinates of frequency and energy level, respectively, and displays a single spectrum analysis in real time. A "B" trace, or lofargram, has horizontal and vertical coordinates of frequency and time, respectively, with line intensity a function of energy level.
Due to the very low signal-to-noise ratios often present in the detected sound, particularly of submarine signatures in the presence of marine life or high sea-state conditions, it is desirable to enhance the spectral signatures of submarines above the background noise. An automatic line integrator (ALI) has been developed for this purpose. Its operation is premised on there being, over a discrete period, relatively high cumulative energy levels of all frequency elements in the spectral signature as compared to the remaining frequency elements associated with background noise. That is, the energy pattern of the spectral signature is not random, while it is random for the noise. Briefly, the ALI biases the analog output of the spectrum analyzer to a reference level equal to the spectrum average, which is the average energy level of all the frequency elements in the analysis, and results in bipolar analog signals. The energy levels of the same frequency elements in a series of such bipolar analog signals are then algebraically summed over a discrete time period for a series of consecutive samples, and the cumulative energy level of each element is displayed. If it is assumed that the energy levels of corresponding noise elements in any series of samples are Gaussian-distributed, then only the energy levels of frequency elements associated with spectral signatures will build up on the display. Since the distribution is not Gaussian, the noise energy levels also build up. Even over a short period of time, the display becomes saturated making the spectral signatures of submarines or the like virtually impossible to detect.